When you turn on the operation of any device, mechanism or device, for some time processes occur in them, which are called non-stationary or starting. The most well-known examples from life - pulling off, say, a loaded truck, train, quite clearly shows that the initial force push is usually required more than the effort in the future.
The same phenomena occur in electrical devices: lamps, electric motors, electromagnets, etc. The starting processes in these devices depend on the state of the working elements: the filament of the lamp, the state of magnetization of the core of the coil of the electromagnet, the degree of ionization of the interelectrode gap in gas discharge lamps , etc. For example, consider the incandescent filament of a lighting lamp. It is well known that in the cold state it has much lower resistance than with
heating to 1000 degrees. in working mode. Try to calculate the resistance
the incandescent filament for a 100-watt bulb is approximately 490 Ohms, and the value measured by the ohmmeter in the idle state is less than 50 Ohms. And now the most interesting thing is to calculate the starting current, and you will understand why the bulbs are lit when turned on.
It turns out that when turned on, the current reaches 4-5 A, and this amounts to a power consumption of more than 1 kW. So why do not 100-watt bulbs burn completely? Yes, just because, when heated, the thread of the bulb has
a growing resistance, which in steady state becomes constant, greater than the initial value and limits the operating current to about 0.5 A.
Electric motors have the widest application in technology, therefore, knowledge of the characteristics of their starting characteristics is of great importance for the correct operation of electric drives. Sliding and torque on the shaft are the main parameters that affect the inrush current. The first relates the speed of rotation of the electromagnetic field with the rotational speed of the rotor and decreases with a set of speed from 1 to the minimum value, and the second determines the mechanical load on the shaft, maximum at the start of start-up and nominal after full acceleration. An asynchronous motor at the time of start-up is equivalent to a transformer with a shorted secondary winding. Because of her small
resistance, the starting current of the motor jumps up to ten times higher than its rated value.
The supply of current to the windings leads to an increase in the saturation of the rotor core with a magnetic field, and the emergence of an emf self-induction, which leads to an increase in inductive
circuit resistance. The rotor starts to rotate, and the slip coefficient decreases, i.e. the engine accelerates. In this case, the starting current with increasing resistance decreases to a steady state.
Problems caused by increased inrush currents occur
due to overheating of electric motors, overload of electrical networks at the moment
start-up, occurrence of shock mechanical loads in connected mechanisms, for example, gearboxes. There are two classes of devices that solve these issues in modern technology - soft starters and frequency converters.
Their choice is an engineering task with the analysis of many operational
characteristics. The load in the actual conditions of use of electric motors is divided into two groups: pump-fan and general industrial. Soft starters are mainly used for loads of the fan group. Such regulators limit the starting current to a level not exceeding 2 nominal values, instead of 5-10 times during normal start-up, by changing the voltage of the windings.
The most widely used in industry are AC motors. However, their simplicity of design and low cost has a flip side - severe start-up conditions, which are facilitated by frequency converters. Especially valuable is the property of frequency
the inverters maintain the starting current of the induction motor for
a long time - a minute or more. The best examples of modern converters are intelligent devices that perform not only the regulation of the start-up process, but also the start-up optimization according to any given operational criterion: the magnitude and constancy of the inrush current, sliding, torque on the shaft, optimal power factor, etc.